AECOM Environment
Upper Fox River/Flint Creek Watershed 7-1 March 2010
7.0 TMDL Approach and Next Stages
This chapter discusses the methodology that may be used for the development of TMDLs for the Upper Fox
River/Flint Creek watershed. In addition to calculating pollutant loads, a site specific implementation plan will
be developed for each TMDL. The implementation plan will provide geographically specific recommendations
that will reduce pollutant loads to impaired waters. While a detailed watershed modeling approach can be
advantageous, a simpler approach is often able to efficiently meet the requirements of a TMDL and yet still
support a TMDL-guided and site-specific implementation plan. The final selection of a methodology will be
determined with consultation with the Illinois EPA based on following factors:
• Fundamental requirements of a defensible and approvable TMDL
• Data availability
• Fund availability
• Public acceptance
• Complexity of water body
A simpler approach shall be used as long as it adequately supports the development of a defensible TMDL. If
it is deemed that this approach will not suffice, a more sophisticated modeling approach will be recommended
for analysis to help better establish a scientific link between the pollutant sources and the water quality
indicators for the attainment of designated uses. Methodology for estimating daily loads will depend on
available data as well as the selected analysis.
7.1 Recommended Modeling Approach for Dissolved Oxygen and pH
Three segments within the Upper Fox River/Flint Creek watershed are targeted for DO TMDL development.
These segments include two Fox River segments and Woodland Lake. Excessive nutrients often result in
algal blooms and extensive rooted plant growths which can deplete oxygen and increase pH. The two main
ways oxygen depletion occurs related to plant growth, both planktonic and rooted, include decomposition and
respiration. Decomposition is the process of breaking down matter. During this process, aerobic bacteria
utilize oxygen to convert organic matter into energy and release carbon dioxide. If the rate of decomposition is
great enough, this process can result in deleterious oxygen depletion. Oxygen is also used during plant
respiration for the conversion of stored sugars into energy. Excessive plant respiration can result in oxygen
depletion. DO concentrations in lakes and ponds are typically at their lowest levels just before dawn after an
evening of respiration without oxygen generation by photosynthesis.
Woodland Lake experienced excessive rooted plant densities historically, but recently the lake experiences
severe algal blooms, either of these conditions can result in oxygen depletion. Given that Woodland Lake
contains excessive TP concentrations which are likely related to low DO, a phosphorus TMDL should be
prepared using available data (see discussion on TP approach). Additional sampling is not required in order to
proceed with this TMDL.
Sources of the lack of DO and elevated pH in the Fox River segments DT-22 and DT-23 have been attributed
to urban runoff, storm sewers, other recreational pollutant source and unknown causes. In addition, flow and
water level manipulation at the Pistakee Lake dam located upstream of DT-23 and the McHenry dam located
upstream of segment DT-22 may be a source for low DO and elevated pH in these segments. Other sources
may include eutrophication, as mentioned above, and delete deleterious inputs.
For the Fox River segments, QUAL2K, a spreadsheet model that is based on the fundamental Streeter-Phelps
DO sag equation, is recommended for DO TMDL development. QUAL2K is a one-dimensional, steady-state
model that can accommodate point and non-point source loading and is capable of modeling DO and pH in
